JP-A-2004-155263 (corresponding to US 2004/0093884 A1) discloses two rotary doors 25 and 26 that are located to open and close a defroster opening 20, a center face opening 21a and a foot opening 22, as shown in FIGS. 17 to 19
The first rotary door 25 opens and closes the foot opening 22 and a communication passage 27a. The defroster opening 20 and the center face opening 21a are provided on a downstream side of the communication passage 27a. The second rotary door 26 opens and closes the defroster opening 20 and the center face opening 21a. 
The rotary doors 25 and 26 include door surfaces 25c and 26c provided at positions separated radially outward from the centers of rotary shafts 25a and 26a by predetermined dimensions. The door surfaces 25c and 26c of the rotary doors 25 and 26 are turned with the rotary shafts 25a and 26a, respectively.
FIG. 17 and FIG. 18 show a foot mode in the related art. FIG. 17 shows a maximum heating state in which a cold air passage 16 is completely closed by an air mixing door 14. FIG. 18 shows an intermediate temperature control state in which the air mixing door 14 is operated to a position with an opening degree.
In the foot mode, when the first rotary door 25 is operated to rotate clockwise by a small angle θ with respect to the completely closed position of the communication passage 27a, a clearance X is formed between the first rotary door 25 and a case-side seal rib 29, so that the first rotary door 25 opens the communication passage 27a by a small amount with the clearance X, while substantially fully opening the foot opening 22.
At this time, the second rotary door 26 fully opens the defroster opening 20 and completely closes the center face opening 21a. Thus, most of conditioned air whose temperature is adjusted by the air mixing door 14 flows into the foot opening 22 to be blown off from foot air outlet ports 24a and 24b toward the passenger's foot area in a passenger compartment.
A part of the conditioned air passes through the clearance X of the communication passage 27a to flow into the defroster opening 20, thereby obtaining defrosting performance of a vehicle window pane. FIG. 19 shows an intermediate temperature control state of a foot/defroster (F/D) mode in which both the communication passage 27a and the foot opening 22 are in a substantially half-opened state by the first rotary door 25.
In the vehicle air conditioner, side face openings are normally set to be located on both left and right sides of the center face opening 21a. The side face openings are always opened in any air outlet modes. This is for the following reason.
Side face air outlet ports supplied with the conditioned air through the above-described side face openings are provided on both left and right sides of a vehicle dashboard (instrument panel). Thus, cool air is blown off from the side face air outlet ports in the cooling operation in summer season, and in addition, warm air is also blown off from the side face air outlet ports toward a passenger's upper body on the window side in a heating operation in winter season.
This can solve a problem that the heating is lacked because the passenger's upper body on the window side (around the shoulder on the window side) receives cool radiation from the window pane on the side of the vehicle to feel cold at chilly time in winter season. Further, the blowoff of warm air from the side face air outlet ports in the heating operation in winter season can obtain a defrosting effect of the window pane on the vehicle side.
In the foot mode or foot/defroster mode which is used as an air outlet mode in the heating operation in winter season, in addition to the foot opening 22, the defroster opening 20 and the side face openings are simultaneously opened, so that the warm air is blown off to the passenger's foot area, the vehicle windshield, and the passengers upper body on the window side at the same time.
At this time, in order to ensure the comfort of a heating feeling and required defrosting performance, it is important that a ratio of an amount of air blown from the defroster opening 20 to an amount of air blown from the side face opening is set to an appropriate value. For example, when the defroster blown-air amount is too much and the side face blown-air amount is excessively small, the blown air from the defroster opening 20 burns the passenger's face, which causes a shortage in heating (feel of coldness) to the passenger's upper body on the window side, resulting in degraded heating feeling.
Conversely, when the defroster blown-air amount is excessively small and the side face blown-air amount is too much, the defrosting performance of the vehicle windshield becomes short, and the feel of heating at the passenger's upper body becomes excessive, resulting in deterioration in heating feeling.
However, the first rotary door 25 serves only as a mechanism for opening and closing the foot opening 22 and the communication passage 27a. For this reason, the use of the first rotary door 25 cannot serve to adjust the ratio of the defroster blown-air amount to the side face blown-air amount.
When a top position (rotational angle θ) of the first rotary door 25 is changed due to air pressure applied to the first rotary door 25 in the foot mode, the clearance X is changed. This may lead to variations in ratio of the blown air amount on the defroster and side-face side to the blown air amount on the foot side. Thus, it is difficult to keep the ratio of the defroster and side face blown-air amount to the foot blown-air amount to a preset appropriate value.
In the intermediate temperature control state in the foot mode shown in FIG. 18 and in the foot/defroster mode shown in FIG. 19, the opened part of the communication passage 27a is located in vicinity of the cold air passage 16. On the other hand, the foot opening 22 is located in vicinity of the outlet side of a warm air passage 18 downstream of a heater core 15. The cooled air in the cold air passage 16 is likely to flow to the defroster opening 20 side. In contrast, to the foot opening 22 side, the warm air in the warm air passage 18 is likely to flow.
Thus, a difference between a temperature of the blown air on the defroster and side-face side and a temperature of the blown air on the foot side (a difference between the upper and lower blown-air temperatures) becomes excessively large. Thus, the excessive decrease in blown air temperature on the defroster/side face side results in degradation in defrosting performance, which causes shortage of heating of the passenger's upper body on the window side. The excessive increase in blown air temperature on the foot side disadvantageously leads to degradation in comfort to the passenger.